FIG. 36 is a circuit diagram illustrating a prior art I-V measurement circuit, a kind of apparatus for testing a semiconductor element. In the figure, reference numeral 1 designates a semiconductor element, such as a GaAs FET and an Si FET, as a target of testing. A source of the semiconductor element 1 is connected to a ground 6 of this measurement circuit. Reference numeral 23 designates a DC power source, and the negative pole of the DC power source 23 is connected to the source of the semiconductor element 1. Reference numeral 30 designates a DC power source, and the negative pole of the DC power source 30 is connected to the source of the semiconductor element 1. Reference numeral 31 designates a current measuring apparatus, such as an ammeter, which is connected between a drain of the semiconductor element 1 and the positive pole of the DC power source 23. Reference numeral 32 designates a current measuring apparatus, such as an ammeter, which is connected between a gate of the semiconductor element 1 and the positive pole of the DC power source 30.
Bias voltages from the DC power sources 30 and 23 are respectively applied to the gate and the drain of the semiconductor element 1 as the target of testing. The current flowing through the gate is measured with the current measuring apparatus 32. Then, a change in the drain current flowing through the semiconductor element 1 due to a change in the bias voltage applied to the gate is measured with the current measuring apparatus 31. Consequently, I-V measurements of the target of testing by inputting a continuous wave (hereinafter referred to as CW) are performed.
By measuring I-V characteristics with the CW input, it is possible to measure the current-voltage characteristics of the semiconductor element (FET) in a stable state from the power that is obtained from the product of current flowing through the semiconductor element, and voltage, i.e., the condition in which the current decreases due to heat generation, and in a state in which electronic charges are stable in a depleted layer in the channel and a surface-depleted layer.
The prior art I-V measurement circuit is arranged as shown in FIG. 36 and the I-V measurements are performed with the CW input operation, so that the input is continuously applied to this circuit. Therefore, the I-V characteristics vary because of self-heating, failing to produce accurate I-V measurements.
In addition, in a recess of a GaAs FET, surface energy levels that adversely affect FET characteristics are produced. In the case of pulsed operation, there is a difference between the speed of electrons flowing through the channel and the speed of electrons at the surface, whereby the characteristics vary. In CW operation, however, since the surface charges are in a stable state, the surface levels do not vary, and no influence of the surface levels is produced. As a result, the I-V characteristics cannot be measured, taking into account the influence of the surface levels.
Further, it is impossible to obtain the I-V characteristics considering an RF swing along a load line in large signal operation of a high-power output FET.